O.,
Hi! My husband and I have taken many LONG car trips, and so I've got several ideas.
First, remember that your little guy will probably find a car ride relaxing and will sleep a large majority of the time. But another thing my husband and I found useful is planning. For example, I would nap during the day with my little one and when my husband got home that night we would take off and drive the night route, knowing that our little one would be asleep all night and we wouldn't have to even both trying to keep them entertained. After my husband takes a cat nap then he would drive and I would sleep. I'm sure when you arrive where you are going the grandparents would love to watch the little guy for you while you and your husband catch a few more hours of sleep.
Also, you can purcahse those little travel dvd players for a vehicle and pop in a baby einsten video. I think Walmart has a 3 month return policy on these types of items if you buy it and then find you didn't really use it or something.
There are several toys you can attach to car seats that tend to keep babies occupied, or occasionally you can sit in the back seat. Also, there are some really wonderful kids CDs by little people and baby einstein. My guy ALWAYS calms down to listen to the little people's kids favorite songs CD.
I hope these suggestions help. Have a great holiday and safe travels!
Dez